Examples of an inspection target that must be inspected for detecting a defect include a porous article and the like. A porous article has been widely used for filters, catalyst carriers, and the like. For example, a porous article has been used for an exhaust gas purification apparatus for a heat engine (e.g., internal combustion engine) or combustion equipment (e.g., boiler), a liquid/gaseous fuel reforming apparatus, a service water/sewage purification apparatus, and the like. A porous article having a honeycomb shape has been used as a diesel particulate filter or a high-temperature gas collection apparatus that traps and removes particulate matter contained in a dust-containing fluid (e.g., exhaust gas discharged from a diesel engine).
A porous article used for such purposes traps and removes unnecessary particulate matter when the treatment target fluid passes through the pores formed in the porous article, or allows the treatment target fluid to come in contact with a catalyst supported on the surface of the porous article and inside the pores formed in the porous article, for example. In order to improve the removal/contact efficiency, a porous article in the shape of a thin film or wall is normally formed into a tubular shape, a monolith shape, a honeycomb shape, or the like to increase the contact area with the treatment target fluid. The porous article cannot achieve the desired filtration performance and catalyst carrier performance if a large hole (defect) is formed through the wall (film) of the porous article. A non-porous article may not achieve the desired performance when a hole (defect) is formed in the non-porous article. A defect in a porous article or a non-porous article can be simply inspected by naked eye observation.
However, it may be difficult to inspect a defect in a porous article or a non-porous article depending on the shape of the inspection target (i.e., porous article or non-porous article) and the position and the size of the defect. For example, a honeycomb structure has a number of through holes that are partitioned by a partition wall and formed through the honeycomb structure in the axial direction. The ends of the through holes may be alternately plugged so that particulate matter is trapped and removed by the porous partition wall. A defect in the partition wall of such a honeycomb structure cannot be observed from the outside.
A defect inspection method that includes generating (producing) particles, introducing the generated particles into the inspection target, and applying light with high directivity to the particles discharged from the inspection target so that the light travels near the inspection target to visualize the particles, has been known (Patent Document 1).
FIG. 6 is a schematic front view showing an apparatus used for this method. Particles and air are respectively supplied from a particle source 15 and an air source 17 to form particle-containing air 19. The particle-containing air 19 is supplied from a particle supply means 1 to one end face of an inspection target 11 under a given pressure. The particle-containing air 19 then passes through the inspection target 11, and is discharged through the other end face of the inspection target 11 as particle-containing air 21. A laser beam irradiation means 3 is disposed so that a laser beam 23 travels near the other end face of the inspection target 11. The particle-containing air 21 that has passed through the inspection target is exposed to the laser beam 23.
The particles contained in the particle-containing air 19 and having a particle size equal to or larger than a given particle size cannot pass through the inspection target 11, and are trapped by the inspection target 11. However, when the inspection target 11 has a defect (e.g., cracks or holes), the particles having a particle size equal to or larger than the given particle size pass through the defect. Since the laser beam 23 is scattered to a large extent by the particles having a particle size equal to or larger than the given particle size, a bright spot is formed corresponding to the defect. The defect can be detected by detecting the bright spot using a light detection means 7.
A defect in an inspection target having a shape that does not allow external defect inspection can be detected with high sensitivity by utilizing the above method and apparatus.    Patent Document 1: JP-A-2002-357562